Development of a one-dimensional model of a closed thermosiphon for cooling a spent-fuel pool

Abstract

Using closed thermosiphons for cooling the spent-fuel pools of nuclear power plants requires calculation tools for simulating the thermal performance of long thermosiphons. The ability of the computer program THERESA to simulate the thermal performance of a vertical thermosiphon with water as the working fluid has been investigated. THERESA was used for simulating a cylindrical thermosiphon with a one-meter-long evaporator and a 32-mm inside diameter. The condenser section had the same inside diameter and a length of 0.5 m. Modelling the long thermosiphons of the type required to span the distance from the spent-fuel pool to the atmosphere requires that the effects of hydrostatic pressure in the evaporator and the pressure drop in the vapor between the evaporator and condenser sections be correctly simulated.The input data to THERESA are the outside wall temperatures of the evaporator and condenser. The thermal resistances between these two surfaces consist of conduction through the walls, the thermal resistance due to vapor-pressure drop in the adiabatic section between the evaporator and the condenser, and the various modes of heat transfer in the liquid phase of the working fluid. The heat transfer in the liquid of the condenser was simulated by a mixture of drop-wise and film-wise condensation. The heat transfer in the evaporator was simulated by a mixture of natural convection, nucleate boiling, and film boiling. After simulating the thermal resistances, THERESA solves for the heat-transfer rate of the thermosiphon.The simulation results were compared with data from experiments with steady-state boundary conditions. The trends in the experimentally derived heat-transfer coefficients as a function of heat-transfer rate were used to identify the type of phenomena that dominate the processes in the evaporator and the condenser. Additional insight into the instabilities occurring during these experiments led to new methods for calculating the heat-transfer coefficients in THERESA. The number of various heat-transfer phenomena simulated in THERESA was reduced to those with the trends that matched the trends in the experimental data. The heat-transfer coefficients for those dominant phenomena were modified in order to represent a time-averaged value over an entire instability cycle. Empirical constants for the modifications were obtained from the experimentally derived values.